1. Field of the Invention
This invention relates to microcellular, open-celled, superabsorbent polymer foams, and a method for producing the same. The foams thus produced have exceptionally rapid sorption rates, as they absorb and retain liquid by a combination of capillary action and pore wall swelling.
2. Description of Related Art
Microporous, open-celled foams have garnered much interest recently due to their potential for numerous and varied applications. For example, these materials are useful in multishell fusion target experiments, as filtration media, in controlled release systems, and as artificial skin and blood vessels. Microporous, open-celled foams can also be employed in much simpler consumer applications such as reusable diapers and other personal hygiene devices. These latter uses often depend upon the absorbent capabilities of the foam and rate of sorption, as well as its strength.
Foams can generally be characterized as materials which have numerous fluid-filled cells distributed throughout their mass. The properties of these materials vary greatly, and depend in large part on the degree of interconnectivity of the cells. For example, should it be desired to use the foam as an absorbent, a greater degree of interconnectivity is desired. If the cells in this two phase fluid-solid system are interconnected, the material is termed an "open-celled" foam. Ideally, a foam used for absorbent purposes should have 100% interconnectivity, in which case the material is termed "bicontinuous" or "open-celled." In contrast, closed cell foams have cells which are discrete, having fluid phases which are independent of that of the other cells.
Another characteristic which greatly affects the properties of a foam is the size of its pores. For example, while natural sponge is a well-known absorbent, it cannot be used in products such as diapers because its large, macroscopic pores cannot hold fluids under even the slightest pressure. For a foam to be useful in a diaper, fluid must be retained under a pressure of about 0.5 psi. In order to achieve this level of retention, the pores must be microscopic, since only then will the capillary forces responsible for fluid retention be sufficient to withstand applied pressures at the desired levels. In addition, only microscopic pores will retain fluid in competition with other absorbent materials such as clothing ("wicking"). Thus, microporous foams (0.1-100 .mu.m pores) are desired for absorbent purposes.
Conventional, macroporous (&gt;100 .mu.m pores) polymeric foams can be produced by a number of methods, the most common being a gas dispersion process whereby a gaseous phase is dispersed throughout a liquid polymer phase. The resultant gas-solid state is then preserved either by physical means such as vitrification, or by polymerization and/or crosslinking of the liquid phase. The cell size in these foams, however, is generally 100-200 .mu.m or larger, and thus their usefulness as absorbents is limited. These products do find use as insulation and packaging material.
Microporous (i.e., 0.1-100 .mu.m pores) polymeric foams have generally been produced by phase separation techniques, however these methods are generally only suitable for hydrophobic polymers. For example, polystyrene foams having densities of 0.02 to 0.20 g/cm.sup.3 and cell sizes of 1-20 .mu.m have been produced. Typically, a homogeneous polymer/solvent solution is first prepared. This solution is then permitted to phase separate by either dissolving a nonsolvent for the polymer in the solution, decreasing the temperature to a point below the upper consolute solution temperature (UCST), or both. Most non-aqueous polymer/solvent systems capable of phase separating exhibit an UCST, and these polymers are typically hydrophobic. After phase separation, the temperature is further reduced to either below the freezing point of the solvent or below the glass transition temperature in order to lock in the desired structure. The solvent can then be removed from the porous, polymer structure either by freeze drying or supercritical drying to produce a microcellular foam. Unfortunately, simple evaporation of the solvent may not be employed for these products because large capillary forces at the liquid-vapor interface will cause the structure to shrink or crack, resulting in the destruction of the cells. In addition, although the expensive and tedious procedures of freeze-drying or supercritical drying may be employed, the resulting microporous foam will redissolve when brought into contact with a good solvent and melt when subjected to elevated temperatures.
Thus, there is a need for microcellular, open-celled foams which exhibit superabsorbency and can be readily synthesized from numerous polymer/solvent systems, particularly hydrophilic polymers.